Chromatography is essentially a physical method of separation in which constituents of a test sample in a carrier gas or liquid are adsorbed or absorbed and then desorbed by a stationary phase material in a column. A pulse of the sample is introduced into a steady flow of carrier gas, which carries the sample into a chromatographic column. The inside of the column is lined with a liquid, and interactions between this liquid and the various elements of the sample—which differ based upon differences among distribution coefficients of the elements—cause the sample to be separated into the respective elements. At the end of the column, the individual components are more or less separated in time. Detection of the gas provides a time-scaled pattern, typically called a chromatogram, that, by calibration or comparison with known samples, indicates the constituents of the test sample. An example of the process by which this occurs is described in U.S. Pat. No. 5,545,252 to Hinshaw.
Often, the sample is first obtained using a sampling device, which subsequently transfers the sample to the chromatograph. One means of obtaining a sample and introducing it into a chromatographic column is known as “headspace sampling.” In conventional headspace sampling, sample material is sealed in a vial and subjected to constant temperature conditions for a specified time. Analyte concentrations in the vial gas phase should reach equilibrium with the liquid and/or solid phases during this thermostatting time. The vial is subsequently pressurized with carrier gas to a level greater than the “natural” internal pressure resulting from thermostatting and equilibration. Then the pressurized vial is connected to the chromatographic column in such a way as to allow for the transfer of a portion of the vial gas phase into the column for a short period of time. An example of such a sampling device is disclosed in U.S. Pat. No. 4,484,483 to Riegger et. al. An example of a chromatographic system employing such a sampling device is disclosed in U.S. Pat. No. 5,711,786 to Hinshaw, which describes using a chromatographic injector between the vial and the chromatographic column.
Typically, it is desired to pre-concentrate the analytes in the sample, and occasionally, remove moisture therefrom, prior to introducing the sample into the chromatographic column. Accordingly, as disclosed in U.S. Pat. Nos. 5,792,423 and 6,395,560 to Markelov, these systems will typically include some kind of “trap” for this purpose, which retains the analytes as they are carried through the trap, and which are later released from the trap, usually by heating, and swept into the chromatographic column.
Though various types of traps have been suggested, one particularly advantageous way to perform this pre-concentration (and possible moisture removal) prior to introducing the sample into a chromatographic column is through the use of an adsorbent trap. These traps, which adsorb the analytes and then subsequently desorbed those analytes, do not suffer from the same slow flow rates as on-line traps (such as cryogenic traps) that result from the impedance of the column. When using an adsorbent trap, a carrier gas can be used to first pressurize the vial and then, after pressurization, to carry the sample vapor through the trap, which will adsorb the analytes to be measured, and then vent out of the system, which is the simplest way to extract vapor from a vessel. Accordingly, numerous arrangements employing an adsorbent trap have been employed for the purpose of pre-concentrating the analytes of a sample extracted by a sampling device such as a headspace sampler. Examples of such arrangements are disclosed in U.S. Pat. No. 5,932,482 to Markelov and U.S. Pat. No. 6,652,625 to Tipler.
However, these systems typically suffer from several disadvantages. One problem is that the process is slow. This is due to the fact that the pressure in the vial is proportional to the amount of sample in the vial. Therefore, as the sample vapor elutes from the vial, the pressure drops. Accordingly, the rate of flow decreases, resulting in undesirable vent times. Indeed, trials have shown that a 22 mL vial pressurized to 40 psig with helium can take longer than five minutes to effect a full extraction, even with high initial flow rates.
Another problem with current systems is that, in order to maximize performance with respect to resolution, sensitivity and inertness, it is generally desired to use capillary columns for chromatography. However, with capillary columns, the carrier gas flow rates will be low, so direct injection of large quantities of vapor is not possible. For example, an injection volume of about 100 μL is typical. A typical headspace sample vial, however, usually has a capacity of about 22 mL, with a maximum sample volume about half that. Therefore, only about 1% of the total headspace vapor is actually injected into the column. Accordingly, system sensitivity can be increased nearly 100× over current headspace sampling systems by increasing the percentage of the available headspace vapor that is actually injected into the column. This is particularly useful for applications requiring very low detection limits, such as environmental and other trace-level analysis of Volatile Organic Compounds (VOCs) in a variety of sample matrices.
Yet another problem with current systems is that a headspace sampler is typically used to test a number of large number vials in sequence, and occasionally, a vial may have a leak or for some other reason may not contain the proper amount of sample, leading to erroneous analytical data. Accordingly, it is advantageous to review the pressure decay profiles for these vials—i.e. in the decay in pressure as a function of time. However, as previously mentioned, the pressure in the vial is proportional to the amount of sample present, and thus, a plot of pressure decay as a function of time results in exponential, rather than linear, profiles, making a determination of whether the rate of decay is appropriate difficult.
What is desired, therefore, is a system and method for extracting headspace vapor that reduces the time required to vent fluid through a trap. What is further desired is a system and method for extracting headspace vapor that increases the amount of sample vapor that is actually injected into a chromatographic column. What is also desired is a system and method for extracting headspace vapor that increases the ability to check for leaks in the sample vials and verify that the proper amount of sample is present therein.